A method has been invented, which transmits first and second signals with frequency multiplexing, through modulating an optical signal being intensity-modulated by the first signal with the second signal composed of other signal using an external modulator (Patent Document 1).
However, in an optical transmitting device shown in FIG. 3 of Patent Document 1, a frequency allocation for an AM-FDM signal (a first signal) and an FM-FDM signal (a second signal) are as shown in FIG. 6 of Patent Document 1. In this case, harmonic distortions and intermodulation distortions of the AM-FDM signal may overlap with the FM-FDM signal frequency, and may interfere the FM-FDM signal.
As such, in a frequency division multiplexing system, when modulating the modulated optical signal by the first signal with the second signal using the external modulator to multiplex the first and second signals with frequency-division, a problem arises that the harmonic distortions and intermodulation distortions generated by the first signal can interfere the second signal.
Patent Document 1: Japanese Patent Application Laid-Open No. 6-104867.    Non-Patent Document 1: ITU-T Recommendation, J.185, “Transmission equipment for transferring multi-channel television signals over optical access networks by FM conversion”, February 2002.    Non-Patent Document 2: Shibata et al., “Optical video distribution systems employing FM conversion”, Transaction B of the Institute of Electronics, Information and Communication Engineers, Vol. J83-B, No. 7, pp. 948-959, July 2000.